Planetary imagers are useful for remote sensing of atmospheric compositions, crop assessments, weather prediction and other types of monitoring activities. Monochromatic and multispectral satellite-based, remote sensors are able to measure properties of the atmosphere above the earth, when their detector arrays are properly calibrated for radiometric response.
A method of calibrating the radiance measured by these remote sensors is to create a reference radiation using a known source of spectral irradiance, such as the sun. The radiation from the sun may be used as a reference signal to a diffusive reflector which, in turn, may provide a known radiance to a remote sensor for calibrating its detector arrays.
The output of the detector arrays may be measured as the remote sensor receives the known diffusely reflected energy from the diffusive reflector. This radiance calibration method provides sufficient information to correctly measure and calculate other types of radiance incident on the remote sensor during normal operation, when using the output of the remote sensor, as the remote sensor views the earth or other target of interest.
The spectral reflectance characteristics of the diffusive reflector, or diffuser panel, however, may change with time due to degradation of the diffuser panel. Since the diffuser panel is employed as the reference source, any change, i.e., degradation of the diffusive surface material, results in a distortion in the measurements of the remote sensor.
Currently, a common diffuser used in on-orbit radiometric calibration is reflective in nature. Typically, such diffuser is made from PTFE or Spectralon™. The PTFE is a pressed polytetrafluoroethylene material. The PTFE, for example, may be sold under the trade name of Algoflon®, which is manufactured by Solvay Solexis of Thorofare, N.J. Another type of material used for a diffusive surface is a thermoplastic resin material sold under the trade name of Spectralon™, manufactured by Labsphere of North Sutton, N.H. An integrating sphere lined with the PTFE or Spectralon™, or lined with barium sulfite (BaSO4) may also be used as a diffuser.
The aforementioned diffusers are of the reflective type. There are also transmissive diffusers. Transmissive diffusers may be made from ground or frosted glass; they may also be made from opal glass or small particulate scatterers placed in a transparent matrix. Transmissive diffusers may also be made from screens or pinhole arrays. Still another type of diffuser may be a diffractive diffuser, such as diffractive scatterers formed from micro-lens arrays or holographic material.
All reflective diffusers have disadvantages. One such disadvantage is that reflective diffusers must have a view of the sun at a limited angle, so that the reflected light from the diffuser is seen by the remote sensor. At very high incidence, or exit angle, a reflective diffuser becomes difficult to characterize. Materials, such as BaSO4, are brittle or fragile, which increases the probability of failure due to rocket launch vibrations.
Transmissive diffusers also have disadvantages. For example, ground or frosted glass has a limited spectral range. In addition, this glass is limited to output angles over which the sunlight scatter may occur. Ground or frosted glass also has a tendency to be heavy and fragile, resulting in drawbacks for on-orbit use due to rocket launch vibration risks. Similarly, opal glass or small particulate scatterers in a transparent matrix have a limited spectral range, due to particulate or inclusion size distribution. Since opal is glass, it also tends to be heavy and fragile, resulting in a drawback for on-orbit use, due to launch vibration risk.
Screens or pinhole arrays also have disadvantages, because their regular geometries may cause undesired diffraction effects. Screens are also difficult to calibrate over ranges of angles, due to the three-dimensional nature of the screens, which may cause internal shadowing. Furthermore, pinholes are subject to clogging from extraneous minute particles. Finally, diffractive scattering material, such as micro-lens arrays or holographic diffusers, have deficiencies in that they have a limited spectral range and a limited range of output angles over which the scatter may occur. Since they are made from glass, they tend to be heavy and fragile, resulting in drawbacks for on-orbit use, due to launch vibration risks.
The present invention solves the above disadvantages, by providing a unique transmissive diffuser, as will be described, for on-orbit radiometric calibration.